Over on the plumbing forum, posters who have new builds often want to change their 'stats to smart 'stats.Personally, I put unused cores into a connector block or wago at each end.
The second heating zone may have a simple battery powered, two wire 'stat installed, but the wiring behind it is 3 core + earth.
Several times, the poster has simply taken the spare core (usually grey), connected it to the N of their new mains powered stat and wondered why it doesn't work.
(Equally as common, is when a poster has taken the live return to the zone valve and connected this to the N!).
Of the posts I have helped with, only one has had the spare core connected to the earth terminals in the wiring centre - the rest were terminated separately in Wagos, spare terminals, or just wrapped in tape!
In terms of helping posters wire up stats, I would prefer the spare core to be terminated separately - if it were connected to earth, with a standby power of less than 4W, some of these cheap wired smart stats might actually power up.